Depeche Mode and (not vs.) Marilyn Manson - Personal Jesus

When an artist covers a song and it's almost as good (if not better) than the original.
Marilyn Manson - Personal Jesus - Sweet Dreams Are Made Of This - Tainted Love
Johnny Cash - Hurt
Gary Jules - Mad World
Also check out Susan Boyle's version of Mad World. Her voice is out of this (mad) world.